Etching operations are frequently used in the formation of modern semiconductor devices. Typically, layers within device structures are formed by depositing a thin film layer of material on the semiconductor substrate, forming a photoresist mask on the thin film layer, and then removing the material left exposed by the photoresist mask in an etching step. A number of characteristics of the reactive ion etching (RIE) technique make it better suited to the extreme demands of modern device processing than many of other etching techniques. Because of the high level of directional selectivity achieved with reactive ion etching, its use enables device structures of greater density to be formed. In addition, the relatively low temperatures at which RIE is performed make such process steps compatible with even the later stages of device processing.
The process of forming a photoresist mask and then etching to remove material is repeated many times in the formation of semiconductor devices. Thus, the etching process has a substantial impact on the processing time required to produce semiconductor devices, as well as on the yields for those processes. As with many of the processing steps employed to form modern semiconductor devices, there is a need to reduce the time needed for etching steps so that process throughput can be increased. There is a similar need to improve the reliability and predictability of etching processes.
Reactive ion etching, and the related technique of magnetically enhanced reactive ion etching (MERLE), removes that portion of a thin film layer not protected by a photoresist mask through a combination of physical bombardment, chemical etching and chemical deposition. In these reactive ion etching processes, a chemical etchant from a plasma created above the etching substrate is transferred to and then absorbed onto the surface of the material to be etched. Ion bombardment provides additional energy to bring the absorbed etchant species to a higher energy state and to accelerate the surface reaction. Chemical etchants are typically chosen so that etching occurs predominantly at the exposed portions of the thin film layer rather than on the surfaces of the photoresist mask. Reacted material, which is typically volatile, is removed from the surface of the device through a vacuum exhaust line. An RIE or MERLE system typically uses a single, RF-powered cathode for generating a capacitively coupled plasma. Accordingly, the etching system walls, including the top lid, are typically grounded in such a system to define the extent of the RF field and the region in which a plasma is generated.
Optimizing the reactive ion etching environment typically includes optimizing the operational pressure within the etching chamber. When chlorine chemistry (e.g., based on Cl.sub.2), fluorocarbon chemistry, or other, similar chemistry is used in the etching system, it is desirable to operate at relatively high pressures (between about 100 milliTorr and about 250 milliTorr) to obtain a good balance between the effects of physical bombardment and chemical etching. When chemically strong etchants, such as SF.sub.6 or NF.sub.3, are used, lower pressure operation is desirable. Higher operating pressures can increase both the density of etchants within the plasma and the rate of transport of etchants to the surface of the etching substrate. In many circumstances, higher operating pressures may improve the selectivity between the photoresist mask and the material being etched. Higher operating pressures require a correspondingly higher level of RF power input to maintain a suitable DC bias to obtain good etch profiles. RIE and MERLE systems also exhibit greater stability and reduced etch residues at higher operating pressures.
Two types of cathode structures are typically used to deliver RF power into reactive ion etching systems. The "isolated cathode" structure uses a cathode separated from the chamber walls primarily by insulation. Although this cathode structure is simple and relatively reliable, the RF power input to the cathode may couple to the chamber wall through vacuum, giving rise to an undesirable secondary plasma. A second type of cathode structure places a shielding structure between the cathode and the walls of the etching system. In typical implementations of this "shielded cathode" design, a shield, typically in the form of a grounded cylinder, is placed around the cathode. An insulating cylinder, also typically cylindrical, physically and electrically separates the cathode from the shield. The grounded shield prevents the creation of a secondary plasma between the cathode and the walls of the etching chamber. Under some conditions, however, arcing may occur between the RF-powered cathode and the grounded shield. In particular, high operating pressure and high RF input power often causes arcing, probably because of the high potential drop over the short distance between the powered cathode and the grounded shield. The thinner plasma sheath associated with high pressure, high input power operation may make the generation of a secondary plasma more likely in the narrow space between the RF-powered cathode and the grounded shield.